Polymer additives and additive blends are used to protect polymers from thermo-oxidative degradation, to provide long term resistance to light or heat, neutralize residual catalyst and to enhance performance properties of the finished product. Polymer additives typically come in powder, granule or pellet form. These additives can be added to the polymer during post reactor extrusion operations, or, in the case of solution and slurry polymerizations, can be added to a liquid carrier and mixed with the polymer suspension post reactor.
Polymer additives in powder form can be difficult to handle and feed and in the case of some additives pose a potential health, fire and explosion risk. If the polymer system requires the addition of several components, the additives must be either pre-blended, or the use of more than one feeder is required.
Preparation of non-dusting pellet forms of additive blends solves many of these problems. U.S. Pat. No. 5,240,642 entitled “Process for Obtaining Granular Forms of Additives for Organic Polymers” describes a process for making low-dust granules of an additive blend including a phenol antioxidant and an acid neutralizer processed in the amorphous or molten state including using an extruder. U.S. Pat. No. 5,844,042 entitled “Process for Obtaining Granular Forms of Additives for Organic Polymers” describes granular forms of additive blends prepared by forcing the blend through a die to form strands and then cutting said strands to form pellets. U.S. Pat. No. 5,597,857, entitled “Low-dust Granules of Plastic Additives” describes additive pellets comprising 10-100% calcium stearates. U.S. Pat. No. 6,740,694B2 entitled “Preparation of low-dust stabilizers” describes using a sub-cooled melt of an additive as a carrier liquid for other additives and as well as amorphous versions of stabilizers. U.S. Pat. No. 6,515,052 entitled “Granular Polymer Additives and their Preparation” describes using a solvent in a compaction process to improve the yield and quality of a compacted additive blend including a phosphite. U.S. Pat. No. 6,800,228 entitled “Sterically hindered phenol antioxidant granules having Balance Hardness” by the same inventor describes using a solvent for the preparation of compacted additive blends which include a phenol.
The patents identified above describe low-dusting forms of additive blends that can be more conveniently and accurately fed to post-reactor extrusion operations for addition to a polymer. In solution, suspension or slurry phase polymerization processes, however, additives and additive blends are frequently added to a liquid before being introduced to the post-reactor polymer-liquid slurry. If the additive or additive blend does not completely dissolve in the liquid carrier a slurry may be formed. The uniformity of the slurry may vary depending on the particle sizes of the undissolved additives. Conventional additives and additive blends will form a slurry containing many large size particles. These particles can settle out of the slurry, collect in dead zones of the process and transport equipment, foul filters and cause defects in the finished product